Patent GB-759139-A (GB)
Title: Colour television apparatus
IPC: H04N11/14
Filed: 1952-09-01
Abstract: 759,139. Television. RADIO CORPORATION OF AMERICA. Sept. 1, 1952 [Sept. 11, 1951], No. 21972/52. Class 40 (3). A colour television receiver for use in a system utilizing a composite video signal comprising a first component representative of total brightness and a second component representative of hue and saturation comprises means for deriving from the composite signal colour signals which are substantially free from signals representative of total brightness a primary object of the arrangement described being to effect such separation without the use of frequencyselective filters. In one embodiment, Fig. 1, the composite video signal (including the brightness component and the colour component) derived from video amplifier 12 is impressed on the common control grid 21 for the three electron guns of the three-colour kinescope 15 and is also supplied, via a video amplifier stage 27, to one end of a delay-line 44, 45 (total delay 240 degrees) which is terminated by resistors 46, 47. The other end of the line is coupled, via a driver stage 29, to an oscillator 25 which is controlled, by means of a reactance device 26, a phase comparator 24 and a " burst " separator 23, to operate at the frequency of the colour subcarrier wave. With this arrangement the wave supplied by oscillator 25 is impressed on the control grids of the R.G.B. colour-signal demodulators 41, 42, 43 respectively such that phase coincidence occurs (at the control grids) between this wave and the colour signals, which are modulated on to the subcarrier wave at 120 degree intervals. The colour-representative signals produced in the output circuits of the demodulators 41, 42, 43 are fed to the colour cathodes 18, 19, 20 of the picture tube 15 and to prevent signal components at the frequency of .oscillator 25 from reaching the cathodes the load resistors 48, 49, 50 are shunted by trap circuits 51, 52, 53 respectively. However, there is also developed in these output circuits signal components representing the combined brightress of the different colours and in accordance with the present invention these components are prevented from appearing at the cathodes 18, 19, 20 by the effect of a high-impedance degenerative circuit 54, 55 common to all the cathodes of the demodulators 41, 42, 43. This circuit, which is made to have an impedance of several thousand ohms over the frequency range of from the total brightness signals (0 to 4.5 Mc/s. approximately) tends to maintain the same proportionality in the output signals in spite of normal tolerance variations in the conversion conductance of the three demodulators so that the only factor which can materially change the potential at the anodes of the valves 41, 42, 43 is the relative phase of the wave from oscillator 25 and the colour-representative signals which are in the form of phase modulations of the received colour carrier wave. In a second embodiment (Fig. 3), pentode valves 57, 58, 59 are employed as colour demodulators the wave from oscillator 25 being supplied directly to the parallel-connected suppressor grids whilst the received composite video signal is fed to the control grids directly, via condenser 61 in the case of the red demodulator 57, via delay line (120 degrees delay) 62 to the blue demodulator 58 and via a second delay line 63 (120 degrees delay) to the green demodulator 59. As in the case of the embodiment of Fig. 1, the anode loads of the demodulators are shunted by trap circuits and the parallel connected cathodes are provided with a common degenerative circuit 72, 73 to present signals at the frequency of oscillator 25 and signals representative of total brightness respectively from reaching the cathodes of the picture tube 15.